May 11, 2011

The study compared the record in the Pumacocha sediment core (PC) to various geological records from South America -- Cascayunga Cave (CC), the Quelccaya ice Cap (QIC), and the Cariaco Basin (CB) -- as well as the annual position of the Intertropical Convergence Zone (ITCZ). Credit: U. of Pittsburgh

A 2,300-year climate record University of Pittsburgh researchers recovered from an Andes Mountains lake reveals that as temperatures in the Northern Hemisphere rise, the planet's densely populated tropical regions will most likely experience severe water shortages as the crucial summer monsoons become drier. The Pitt team found that equatorial regions of South America already are receiving less rainfall than at any point in the past millennium.

The researchers report in the Proceedings of the National Academy of Sciences (PNAS) that a nearly 6-foot-long sediment core from Laguna Pumacocha in Peru contains the most detailed geochemical record of tropical climate fluctuations yet uncovered. The core shows pronounced dry and wet phases of the South American summer monsoons and corresponds with existing geological data of precipitation changes in the surrounding regions.

Paired with these sources, the sediment record illustrated that rainfall during the South American summer monsoon has dropped sharply since 1900exhibiting the greatest shift in precipitation since around 300 BCEwhile the Northern Hemisphere has experienced warmer temperatures.

Study coauthor Mark Abbott, a professor of geology and planetary science in Pitt's School of Arts and Sciences who also codesigned the project, said that he and his colleagues did not anticipate the rapid decrease in 20th-century rainfall that they observed. Abbott worked with lead author and recent Pitt graduate Broxton Bird; Don Rodbell, study codesigner and a geology professor at Union College in Schenectady, N.Y.; recent Pitt graduate Nathan Stansell; Pitt professor of geology and planetary science Mike Rosenmeier; and Mathias Vuille, a professor of atmospheric and environmental science at the State University of New York at Albany. Both Bird and Stansell received their PhD degrees in geology from Pitt in 2009.

"This model suggests that tropical regions are dry to a point we would not have predicted," Abbott said. "If the monsoons that are so critical to the water supply in tropical areas continue to diminish at this pace, it will have devastating implications for the water resources of a huge swath of the planet."

The sediment core shows regular fluctuations in rainfall from 300 BCE to 900 CE, with notably heavy precipitation around 550. Beginning in 900, however, a severe drought set in for the next three centuries, with the driest period falling between 1000 and 1040. This period correlates with the well-known demise of regional Native American populations, Abbott explained, including the Tiwanaku and Wari that inhabited present-day Boliva, Chile, and Peru.

After 1300, monsoons increasingly drenched the South American tropics. The wettest period of the past 2,300 years lasted from roughly 1500 to the 1750s during the time span known as the Little Ice Age, a period of cooler global temperatures. Around 1820, a dry cycle crept in briefly, but quickly gave way to a wet phase before the rain began waning again in 1900. By July 2007, when the sediment core was collected, there had been a steep, steady increase in dry conditions to a high point not surpassed since 1000.

To create a climate record from the sediment core, the team analyzed the ratio of the oxygen isotope delta-O-18 in each annual layer of lake-bed mud. This ratio has a negative relationship with rainfall: Levels of delta-O-18 are low during the wetter seasons and high when monsoon rain is light. The team found that the rainfall history suggested by the lake core matched that established by delta-O-18 analyses from Cascayunga Cave in the Peruvian lowlands and the Quelccaya Ice Cap located high in the Andes. The Pumacocha core followed the climatological narrative of these sources between the years 980 and 2006, but provided much more detail, Abbott said.

Delta-O-18 levels from Pumacocha correlate with geological temperature records, including solar radiation levels, titanium concentration at Cariaco Basin, and annual temperatures in the Northern Hemisphere and North Atlantic. Credit: U. of Pittsburgh

The team then established a connection between rainfall and Northern Hemisphere temperatures by comparing their core to the movement of the Intertropical Convergence Zone (ITCZ), a balmy strip of thunderstorms near the equator where winds from the Northern and Southern Hemispheres meet. Abbott and his colleagues concluded that warm Northern temperatures such as those currently recorded lure the ITCZthe main source of monsoonsnorth and ultimately reduce the rainfall on which tropical areas rely.

The historical presence of the ITCZ has been gauged by measuring the titanium concentrations of sea sediment, according to the PNAS report. High levels of titanium in the Cariaco Basin north of Venezuela show that the ITCZ lingered in the upper climes at the same time the South American monsoon was at its driest, between 900 and 1100. On the other hand, the wettest period at Pumacochabetween 1400 and 1820, which coincided with the Little Ice Agecorrelates with the ITCZ's sojourn to far south of the equator as Northern Hemisphere temperatures cooled.

Related Stories

University of Pittsburgh-led researchers extracted a 6,000-year climate record from a Washington lake that shows that the famously rain-soaked American Pacific Northwest could not only be in for longer dry ...

The rain band near the equator that determines the supply of freshwater to nearly a billion people throughout the tropics and subtropics has been creeping north for more than 300 years, probably because of ...

Monsoons, the life-giving, torrential rains of Asia and Africa, have an ancient, unsuspected connection to previous Ice Age climate cycles, according to scientists at the University of California, Santa Barbara, and at Kiel ...

Climatic fluctuations close to the equator show a different pattern to climate change in the Arctic and Antarctic. In the tropics distinct 11500 year fluctuations between wet and dry periods can be clearly identified which ...

Geoscientists at the California Institute of Technology have come up with a new explanation for the formation of monsoons, proposing an overhaul of a theory about the cause of the seasonal pattern of heavy winds and rainfall ...

Recommended for you

An analysis of buildings tagged red and yellow by structural engineers after the August 2014 earthquake in Napa links pre-1950 buildings and the underlying sedimentary basin to the greatest shaking damage, ...

As everyone who lives in the San Francisco Bay Area knows, the Earth moves under our feet. But what about the stresses that cause earthquakes? How much is known about them? Until now, our understanding of ...

(Phys.org)—A trio of researchers with the Indian Institute of Science has found, via computer simulation, that deforestation in one part of the world can impact rainfall patterns in another. In their paper ...

It's no surprise that Arctic sea ice is thinning. What is new is just how long, how steadily, and how much it has declined. University of Washington researchers compiled modern and historic measurements to ...

Reasearchers at the University of Cadiz have carried out a study that establishes the atmospheric conditions responsible for the generation of extreme meteorological events in the Gulf of Cadiz, which can ...

That's right - instead of providing a reasoned argument using logic, facts, and references, just impugn them using guilt by association. They may have received Federal Money, so they must be lying.

Now lest we forget, weren't you a NASA P.I. that later received government money while teaching at the University of Missouri-Rolla? Did the government just happen to switch from being good to being evil at the precise moment you stopped receiving their money, or were you were happy to work as the tool of an evil government as long as you received a paycheck?

Did the government just happen to switch from being good to being evil at the precise moment you stopped receiving their money?

The government is not evil. Nor was President Eisenhower.

He simply warned of the danger that government research agencies would tend to promote the concepts and findings that are fashionable with the politicians who decide how much funds the research agency will get for the next year.

Scientists and directors of federal research agencies are no more evil than the dogs Pavlov trained with dog biscuits instead of research funds.

I agree with barakn - yet again a scientific study is attacked - of course with no counter arguments or data - just wild conspiracy theory about the grand collaboration of scientists. The problem for me is how high the stakes are. The scientists are doing there best to study the situation - and they are very concerned. We all win if we get a handle on the situation - and find solutions. If the problem is as bad as it seems - there is going to be a lot of pain to go around. It is frustrating to me that a great web site (physorg) - dedicated to the reporting of science - has to deal with so much antiscience. It would seem to me that if you distrusted scientists so badly - you would be reading a different kind of web site.

I agree with barakn - yet again a scientific study is attacked - of course with no counter arguments or data - that if you distrusted scientists so badly - you would be reading a different kind of web site.

1. President Eisenhower warned on 17 January 1961:

"The prospect of domination of the nations scholars by Federal employment, project allocations, and the power of money is ever present and is gravely to be regarded."

The choice presented to the public by environmentalists the UN and supported by governments worldwide has been presented as either making drastic changes with massive economic implications, or condemning the world to climatic catastrophe.

In effect the climate radicals offer/demand was no choice at all!

If you choose to condemn the world to catastrophe by not excepting alarmists constant predictions of CAGW (based on climate models and faith) you are condemned as a denier or a right wing bigot, with evil intentions? If you reject the drastic changes demanded by environmentalists with massive economic implications you were condemned as advocates of evil.

Alarmist has painted themselves into a corner by demanding complete acceptance without question of a flimsy manufactured science without credible evidence, based on climate changes that have always occurred throughout the history of the planet.

By offering nothing but extreme sacrifice without a positive outlook good or bad the choices were zero or nothing and became totally unacceptable, thats why the public are no longer buying into the AGW hoax!

You have discredited science especially so called climate science. You will continue to publish the same repackaged doom and gloom pronunciations as your audience shrinks to the party faithful and the fanatics. You are reduced to fighting a rearguard action and will soon become a bad but expensive footnote in history!

A quote from Eisenhower's address omatumr - "Yet, in holding scientific research and discovery in respect, as we should" Seems you cherry pick your information. Have you looked at the science that contradicts your position that the sun is responsible for the current warming trend? I'll get you started - http://solar-cent...AQ2.html Look hard at the chart presented there - showing sun spot activity, global temperatures, and C02 levels. Does it at least make you wonder?

The variable Sun is the most likely candidate for the natural forcing of past climate changes on time scales of 50 to 1000 years. Evidence for this understanding is that the terrestrial climate correlates positively with solar activity.

Clearly there are idiots on both sides of these arguments. I agree that outlandish predictions not based on science have hurt those trying to determine the scientific realities. I also know that most of those who don't believe anything out of the ordinary is happening lack a basic understanding of radiative heat transfer. Then there are those on this blog that spout religion as the reason the science has to be incorrect.

The bottom line is that there are idiots on both sides but there are very few scientists (and none of general acclaim) who deny that humans are causing the planet to get warmer due to GHGs. The scientists walk the middle ground with incompetents to each side. As an example, Omator embarrasses himself every time he pulls out his Neutron star theory. Please just focus on the topic - which in this case is that we will have more severe tropical droughts. This was a well done study. Take a shot at the study or shut up. Don't get so far off topic.

As a professional skeptic (engineer), I don't put much stock in the accuracy of future climate predictions based on the recent (<3k years) climate record. More data is better, but it all has to be evaluated in context. Predicting hard future climate data based on current findings is like trying to guess someone's IQ by doing an autopsy.

I do see a lot of arm-waving and screaming from the AGW crowd, while credible dissenters are studiously ridiculed or ignored. This is ceasing to be science and becoming a religion.

From the very top of the cited list, we read ""Global warming results not from the emission of greenhouse gases into the atmosphere, but from an unusually high level of solar radiation and a lengthy almost throughout the last century growth in its intensity."

ROTFLMAO.

Please sign in to add a comment.
Registration is free, and takes less than a minute.
Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.

Javascript is currently disabled in your web browser. For full site functionality, it is necessary to enable Javascript.
In order to enable it, please see these instructions.